It is well known in the art to provide agricultural harvesting machines, such as forage harvesters, with an apparatus for detecting foreign material in the stream of crop material which is being fed to a crop processing unit, such as a rotating cutterhead co-operating with a stationary shear bar. Such detector apparatus may be a metal detector of the type described in EP-B-0,102,665. The signal generated by this detector is fed to feeder arrest means, which provoke an immediate stop of the means feeding the crop material to the cutterhead. Thus is prevented that stray metal objects, which were picked up from the field, reach the cutterhead and cause serious damage to the knives and the shear bar. The arrest system also prevent that smaller metal particles are comminuted by the cutterhead and mixed with the crop which is fed to the cattle.
Typical mechanical arrest means as disclosed by U.S. Pat. No. 5,921,071 and U.S. Pat. No. 4,296,591 comprise a ratchet wheel which is mounted to the drive line of the feedrolls of the feeder means and a pawl which is positioned to engage this ratchet upon detection of metal. The movement of the pawl is controlled by a solenoid which is operated by the foreign material detector. Actuation of the pawl has to be combined with the simultaneous deactivation of a clutch in the drive line of the feedrolls to prevent damage to the engine or components of the drive line.
The use of such clutch is not required when the feeder means are driven by a hydrostatic motor as described in EP-A-0,848,902. The hydraulic circuitry then may be equipped with an arrest valve which is operable to cut the oil flow to or from the motor upon detection of a foreign object as suggested by FR-A-2,173,352. Such systems may work satisfactorily when used on the former low capacity forage harvesters. However, it has been experienced that serious problems as to reaction. speed and life time emerged when such systems are applied to the present-day high capacity harvesters in which the rotating feedrolls have a greater kinetic energy. Especially the low reaction time is critical as the foreign object may have the time to reach the cutterhead.
It therefore is an object of the present invention to provide a drive and arrest system for the feeder means which, on the one hand, does not require a clutch which has to be disengaged upon detection of a foreign object, and, on the other hand, is sufficiently powerful and fast to halt the crop flow before the foreign object reaches the crop processing means, even on larger harvesters.